In the field of manufacture of polyolefins (e.g. polyethylene), much attention has been devoted to finding new and improved catalysts capable of producing olefin polymers having improved properties.
For many applications, it is desirable to produce polyolefins having high melt flow index property. In the past, it has been found that high melt index was not achievable by use of simple chromium-silica catalysts. Therefore, producers of polyolefins turned to the use of titanium-containing catalysts such as Tixe2x80x94Cr-silica tergel or Ziegler-Natta catalysts. These alternatives to chromium-silica catalysts are generally more expensive than chromium-silica catalysts. Further, Ziegler catalysts are often difficult to obtain in a supported form. Also, Ziegler catalysts can result in the introduction of undesirable amounts of chlorine (and other constituents) into the polyolefin.
Attempts have been made to increase the melt index obtainable using chromium silica catalysts by use of high pore volume silica gels or by adding other substances to the gel such as phosphorus-containing additives. The success of these attempts has been very limited. Accordingly, there remains a need for alternative catalysts capable of producing high melt index polyolefins without the above-mentioned disadvantages.
The invention provides new chromium silica catalysts which are capable of achieving high melt index polyolefins without the use of titanium or other additives. The catalysts of the invention are capable of achieving melt index in excess of 4.0 as measured by ASTM D1238-79, Condition E, Procedure B. The invention also encompasses methods for making the catalysts of the invention.
In one aspect, the invention encompasses catalyst compositions suitable for polymerization of olefins wherein the catalysts comprise:
(a) an amorphous silica gel support having a pore volume of 1.9-2.9 cc/g, a surface area of about 250-900 m2/g, and a pore size distribution wherein at least 40% of the pores have a pore size of about 200-400 xc3x85, and
(b) chromium,
wherein the catalyst is capable of yielding a polyolefin material having (1) a melt flow index of at least about 4.0 in a slurry polymerization of ethylene wherein the catalyst composition is the only catalyst employed, the melt flow index being determined by ASTM D1238-79, Condition E, Procedure B.
In another aspect, the invention the invention encompasses a process for manufacture of a chromium-containing silica catalyst, the process comprising:
(a) combining an alkali metal silicate solution and a mineral acid to form a mixture having pH of 6-10 and a silica content of about 16-21 wt. % and maintaining the mixture at a temperature of about 10-20xc2x0 C.,
(b) forming the mixture into beads and rapidly gelling the beads,
(c) washing the gelled beads in an aqueous bath at pH=3-6 for about 15-30 hours,
(d) aging the beads in an alkaline bath of pH=8-10 for about 5-25 hours,
(e) immersing the washed aged beads in an organic solvent suitable for azeotropic distillation and removing water from the beads by azeotropic distillation,
(f) drying the beads.
A chromium compound may be added to the mixture to be gelled, to the gelled beads before or during step (e) or to the dried beads. Preferably, the beads are formed by injecting the mixture into air. The beads preferably gel instantaneously when they are formed.
These and other aspects of the invention are described in further detail below.
The invention encompasses the discovery that certain silica gels when used in chromium silica catalysts result in a capability of producing high melt index polyolefins. The invention further encompasses the discovery of processes for making these chromium silica catalysts.
The silicas useful in the invention are characterized by a combination of high pore volume, moderately high average pore diameter and narrow pore size distribution. The silicas of the invention are believed to be primarily produced by instantaneous gelation processes, and more particularly by so-called bead gel processes.
The silica gels and chromium silica catalysts of the invention are preferably have a pore volume of at least about 1.9-2.9 cc/g, more preferably about 2.2-2.7 cc/g. The gels and catalysts are further characterized by an average pore size of preferably about 200-500 xc3x85, more preferably about 250-350 xc3x85. The silica gels and resulting catalysts preferably have a pore size distribution wherein at least 40% of the pores have a pore size of 200-400 xc3x85, more preferably about 50-70% of the pores have a pore size of 250-350 xc3x85. Preferably, the silica gels and resulting catalysts have at least about 80% of total surface area in pores above 140 xc3x85; more preferably at least 95% of total surface area in pores 150-450 xc3x85 range where the pore size distribution is measured by Nitrogen Desorption (BET) Method.
The catalysts of the invention comprise chromium and silica. The catalysts may contain other additives, however the catalysts preferably consist essentially of chromium and silica. In any event, the catalysts preferably do not contain titanium.
The chromium silica catalysts of the invention may contain any catalytically effective amount of chromium. Preferably, the amount of chromium is about 0.1-2.0 wt. % based on the total weight of the catalyst, more preferably about 0.5-1.3 wt. %. The average particle size of the catalyst is preferably about 30-300 microns, more preferably about 80-150 microns.
The catalysts of the invention are further characterized by their ability to achieve a melt flow index of at least about 4.0, more preferably about 5-7 as measured by ASTM D1238-79, Condition E, Procedure B. The above mentioned properties are measured with respect to polymerization of ethylene in a slurry polymerization process.
The chromium silica catalysts of the invention are preferably made using a modified bead gel process. The process preferably comprises:
(a) combining an alkali metal silicate solution and a mineral acid to form a mixture having pH of 6-10 and a silica content of about 16-21 wt. % and maintaining the mixture at a temperature of about 10-20xc2x0 C.,
(b) forming the mixture into beads and rapidly gelling the beads,
(c) washing the gelled beads in an aqueous bath at pH=3-6 for about 15-30 hours,
(d) aging the beads in an alkaline bath of pH=8-10 for about 5-25 hours,
(e) immersing the washed aged beads in an organic solvent suitable for azeotropic distillation and removing water from the beads by azeotropic distillation, and
(f) drying the beads.
A chromium compound may be added to the mixture to be gelled, to the gelled beads before or during step (e), or to the dried beads.
The alkali silicate is preferably a sodium silicate. The alkali silicate solution preferably has a concentration of about 22-26xc2x0 Baume (Bxc3xa9). The mineral acid is preferably sulfuric acid. The acid preferably has a concentration of about 16-19xc2x0 Bxc3xa9. The silicate and acid are preferably combined in proportions to yield a mixture containing about 18-21 wt. % silica and a pH of about 6-10.
The silicate and the acid are preferably maintained at a relatively cool temperature prior to mixing. Preferably the temperature is kept at about 10-20xc2x0 C. Since the mixture will tend to gel rapidly, in-line mixing techniques such as those described in U.S. Pat. No. 3,872,217 or Japanese Patent 48-13834 are preferably used to form the beads. The gelation preferably occurs within 3 seconds from when the mixture is formed, more preferably within 0.5 seconds.
The gelled beads are then collected as they fall in an aqueous bath. The bath preferably has a pH of about 2-6. Preferably, the bath contains sulfuric acid. The soak time in the bath is about 10-60 minutes. The beads are then washed in an acid wash (pH=3-6) for about 15-30 hours, preferably about 20 hours. The washed beads are then ammonia aged at pH of about 8-10 for about 5-25 hours, preferably about 6-20 hours. The end level of Na2O in the beads is preferably less than 0.1 wt. %, more preferably less than 0.05 wt. %.
The washed beads are then treated to remove the water from the pores. Preferably, water removal is accomplished by azeotropic distillation. Where the water is removed by azeotropic distillation, the hydrogel is simply combined with an organic solvent suitable for azeotropic distillation, such as hexanol. The distillation is preferably conducted at about 95xc2x0 C.-170xc2x0 C. The distillation is typically performed until all the water is removed from the gel. The resulting xerogel can then be dried and calcined to remove the organic solvent. Preferably, the drying is performed at 170-250xc2x0 C.
The addition of chromium to the silica gel may be performed at one or more places in the above process. For example, the chromium can be added to the silica hydrogel before water removal using an aqueous solution of a chromium compound. The chromium can also be added during the azeotropic distillation. The chromium can also be added by post-impregnation of the calcined silica gel. The method of chromium addition is not believed to be critical to the results of the invention. Conventional chromium compounds such as chromium acetate, chromium acetyl acetonate or chromium nitrate may be used. The chromium-containing catalyst may be calcined as appropriate to achieve an activated catalyst.
The catalysts of the invention are believed to be suitable for a variety of polymerization reactions. The catalysts are especially useful for the production of polyethylene. The utility of the catalyst of the invention is not limited to any specific polymerization technique.
The invention is further illustrated by the examples below. It should be understood that the invention is not limited to the specific details of the examples.